This invention relates to an apparatus and method for analyzing an optical signal and, more particularly, for analyzing an optical spectrum of a Dense Wave Division Multiplexing (DWDM) system or a Frequency Division Multiplexing (FDM) system sing a tunable optical filter.
Dense Wave Division Multiplexing (DWDM) is widely used in fiber optic transmission systems to expand the capacity of the fiber optic system. In a DWDM network a plurality of optical channels, each operating at a specific wavelength are transported in single fiber. Each wavelength is separated by a channel spacing in the order of, for example, 0.4 nm. As many as 160 channels are transmitted over a single fiber.
A necessary part of network management includes performance monitoring to guarantee the quality of service. Conventional link performance monitoring (LPM) is performed in the transport layer of the network in the electronics domain and on a per-channel basis. The use of DWDM necessitates optical domain performance monitoring, which measures the optical signal-to-noise-ratio, wavelength, power of each channel and other characteristics of each channel. Traditional diffraction-grating-based Optical Signal Analyzers (OSAs) are generally large in size in order to achieve a reasonable optical resolution. Photo-diode array-based OSAs are compact in size but they generally provide poor spectral resolution.
As shown in prior art FIG. 1, a depiction of a prior art analyzer 100 indicates the use of a reflective grating 102 receiving a parallel light beam from a collimator 104 and refracting that light beam to a photo diode array 106. The photo diode array 106 detects the amplitude of light signals 108 and converts them into electronic signals 110 for receipt by the signal processor 112 for processing. A data processing module 114 provides for data acquisition and processing.
Typically, the analyzer 100 provides low spectral resolution. The increase of resolution has two fundamental limitations. First, the limited number of photo diodes in the photo diode array negatively affects resolution. As the performance monitoring is provided over the 256-element photo diode array 106, only 256 effective samples are taken over the entire spectral range. The resolution may be increased by implementing a 512-element or greater photo diode array 106, but this increase in elements presents challenges in technology and manufacturing. Hence, additional elements in the photo diode array 106, in order to improve resolution, become cost prohibitive.
Second, the resolution of the reflective grating 102 is not fine enough for high resolution of the analyzer 100. For example:
N=xcex/xcex94xcex,where:
N=Number of lines in reflective grating,
xcex=Wavelength, and
xcex94xcex=Reflective grating resolution.
If the prior art analyzer 100 is desired to have a reflective grating resolution of xcex94xcex=0.01 nm, and with a wavelength of 1550 nm, then:
N=xcex/xcex94xcex=1550 nm/0.01 nm=155,000 lines.
If the density of the reflective grating 102 is 600 lines/mm, then the size of the reflective grating is 155,000/600 lines/mm=258 mm which is not a feasible solution to increase resolution for a small spectrum analyzer.
Therefore, a solution is needed that would provide for a wide bandwidth monitoring and a better wavelength resolution.
There is, therefore, provided in the practice of the invention a novel optical domain signal analyzer, for providing high resolution spectrum analysis over a wide optical bandwidth. The optical domain signal analyzer broadly includes an optical filter for providing wavelength samples of a received optical signal, a dispersing element for receiving the samples and dispersing the samples, a detector for receiving the dispersed signal and for providing electrical signals representative of the dispersed sample, and a processor for receiving the electrical signal and calculating the characteristics of the spectrum. In a preferred embodiment, the detector is a linear imaging sensor. Also, in a preferred embodiment, the optical filter is a Fabry-Perot interferometer (FPI) comprised of two parallel mirrors, each of which only partially transmit light. If constructive interference occurs in the FPI cavity, then the light with that particular frequency is transmitted from the FPI.
Accordingly, it is an object of the present invention to provide an improved optical domain signal analyzer for providing for high resolution and wide bandwidth testing of an optical signal.